The invention relates to a semiconductor device having a substrate on a first side of which there are provided a first semiconductor element and at least one security coating which comprises a powdery filler incorporated in a matrix.
The invention also relates to a smartcard provided with a semiconductor device comprising a memory and a security coating which comprises a powdery first filler incorporated in a matrix.
U.S. Pat. No. 5,399,441 discloses a semiconductor device of the type mentioned in the opening paragraph wherein the security coating is a layer having a matrix of silicon oxide in which a powdery filler is incorporated. When the matrix is filled with a filler, such as an oxide, a nitride or a carbide of silicon, aluminum or a metal, the coating inhibits at least 90% of the transmission of visible light. When the matrix is filled with an inorganic salt of a heavy metal, the coating inhibits at least 90% of the transmission of a specific type of infrared radiation.
It is a drawback of the known semiconductor device that about 10% of the radiation of selected wavelengths is still transmitted through the coating. Considering that state-of-the art microscopes operate with visible light or near infrared radiation, this percentage is too high to provide adequate security. Personal or financial data contained in the known semiconductor device will be accessible and a method to change the data illegally might be found by reverse engineering of the semiconductor device. So if the known semiconductor device is applied in a smartcard and if the smart card falls into the hands of a dishonest person, the data therein will be vulnerable to a breach of security.
It is, therefore, a first object of the invention to provide a semiconductor device of the type mentioned in the opening paragraph wherein the coating inhibits transmission of visible light and of near infrared radiation to a larger extent. It is a second object to provide a smart card of the type mentioned in the opening paragraph whose semiconductor device has such an improved coating.
The first object is achieved in that:
the difference between the refractive index of the powdery first filler and that of the matrix is at least 0.3, and
the coating comprises a second filler which is a substantial absorber of radiation of wavelengths at least in the range of from 800 to 1400 nm and is free of heavy metals.
The second filler is a material, for example, in the form of particles which transforms incident radiation into heat. As a substantial absorber of radiation, it absorbs the radiation for at least 99 percent. The second filler absorbs the radiation to at least 1400 nm. Radiation of a wavelength larger than 1.4 xcexcm need not be absorbed as much as 99%, as the large wavelength itself hides details of the first semiconductor element in the semiconductor device of the invention. Preferably, the second filler absorbs the radiation of wavelengths in the range of from 800 to 2000 nm. Suitable materials are, for instance, titanium nitride and titanium oxynitride.
The second filler in a preferred embodiment comprises titanium nitride. Due to the use of titanium nitride as a second filler, of the transmittance of at least 99.9% of radiation of a wavelength of between 600 and 2000 nm can be easily inhibited. The inventors have found that particles of TiN in the coating can have a surface comprising TiO2, which is believed to follow from accidental oxidation. Such oxidation occurs, for example, when the security coating is applied during the manufacture of the semiconductor device from an acid, aqueous composition.
A first advantage of this second filler is that titanium nitride is known to be used in the manufacture of semiconductor devices in cleanroom facilities. Moreover, it is free of heavy metals. This is an important advantage as heavy metals are poisonous materials that present problems both during the production and as waste after disposal of the semiconductor device comprising the security coating. A further advantage is that titanium nitride is commercially available in various particle sizes.
The first filler present in the security coating of the device of the invention operates by scattering visible light. Radiation in the visible spectrum as well as in the ultraviolet spectrum is thus inhibited from penetrating by scattered reflection. In order to ensure that the coating has sufficient scattering potential, it was found that the difference between the refractive index of the first filler and that of the matrix should at least be 0.3. As the refractive index of a matrix is generally of the order of 1.4-1.5, first fillers with a refractive index larger than 1.7-1.8 can be used. Examples of such first fillers include oxides of zirconium, titanium, zinc, manganese, chromium, niobium, iron, nickel, strontium, yttrium, vanadium, gallium, copper and cobalt and nitrides of niobium, titanium and zirconium.
The first filler in a further embodiment comprises titanium oxide. This material has a very high refractive index and causes, therefore, intense scattering of visible light and of radiation in the ultraviolet spectrum. If titanium nitride is used as the second filler, it is preferred to have a weight ratio of titanium oxide and titanium nitride in the range of from 0.25 to 4 in the coating.
The amount of filler used in the coating of the semiconductor device of the present invention can be varied over a wide range, depending, for example, on the electrical characteristics desired in the coating. Generally speaking, the first and the second filler are present in a combined amount in the range of from 10 to 90 percent by weight of the coating. The first and the second filler are preferably present as particles of a size in the submicron range.
The material of the matrix of the coating can be chosen from several materials, such as a mono(metal)phosphate compound wherein the metal is, for example, chosen from the group of zinc and aluminum, or a component prepared from a silica precursor resin. Such matrices are described, for example, in the non-prepublished application WO IB99/01007 and in U.S. Pat. No. 5,399,441. Preferably, monoaluminum phosphate is used for the matrix. As a result of the use of this material, the security coating has a great mechanical strength and a good etching persistency. Furthermore, the coating with monoaluminum phosphate can be readily provided in a thickness ranging from 1 to 10 xcexcm.
In a preferred embodiment the coating has a thickness in the range of from 1 to 5 xcexcm. It is especially preferred to have a coating thickness of less than 3 xcexcm. The coating is nevertheless opaque to visible light and near infrared radiation. A first advantage is that the thickness of the semiconductor device is only slightly increased by the provision of the security coating. This is advantageous for the use of the semiconductor device of the invention in a smartcard in which the device thickness is limited. A second advantage is that it is also possible to apply more than one coating in the semiconductor device. A third advantage is that the application of end-contacts in the manufacture of the semiconductor device is easier than in the case of coatings of larger thickness. These end-contacts can be applied by way of a method successively comprising the steps of depositing a photoresist in regions where end-contacts are desired; depositing and drying a composition comprising fillers incorporated in a matrix; removing the photoresist and the composition deposited thereon; heating so as to form the security coating; and applying an electrically suitably conducting material, such as copper, in said regions. Using a coating thickness of less than 5 xcexcm facilitates the removal of the photoresist and the coating deposited thereon.
If desired, other materials may also be present in the coating. For instance, an agent which modifies the surface of the fillers for better adhesion or a dispersion agent may be used.
A further embodiment of the semiconductor device of the invention comprises a light-sensitive element which is covered by the security coating and, after damaging of the security coating, reacts to exposure to visible light by inducing a permanent change of state of the element containing data to be protected. Said reaction, taking place in this embodiment after malicious damaging of the coating, can be of an electrical or chemical nature. The layout of selected parts of the semiconductor device, such as a layer comprising interconnect lines, is thus destroyed.
Another embodiment of the semiconductor device of the invention comprises a light-sensitive element and an electrically programmable element containing data to be protected, which elements are covered by the security coating and which light-sensitive element, after damaging of the coating, reacts to exposure to visible light by inducing erasure of the data to be protected and by bringing the electrically programmable element into a non-programmable state.
Said light-sensitive elements comprise, for example, resistors in combination with diodes or transistors which are coupled to an output terminal, such as an amplifier. An example of a light-sensitive element is known for EP-A 0939 933. It comprises a biasing transistor arranged to provide a bias current, a reverse biased transistor with a control electrode arranged to be reverse biased by said bias current and with a conducting electrode, and also a resistor coupled between a supply voltage and said conducting electrode. Incident visible light is detected by a voltage drop at the conducting electrode of the reverse biased transistor which can then apply a signal to the output. The output can induce the resetting of an electrically erasable programmable random-access memory known as EEPROM. The output can also induce complete writing of a one-time electrically programmable random-access memory. The complete writing causes erasure of the data to be protected.
The substrate of the semiconductor device of the invention can be made from several materials. Suitable materials are, for example, silica, alumina and a polymer material such as polyimide. There are several materials and constructions which can be used in the first semiconductor element of the semiconductor device of the invention. Known constructions are, for example, the MOS field effect transistor and the bipolar transistor. Such a construction can comprises memory units as is known per se. Known materials comprise metals, inorganic semiconductors and organic conductors and semiconductors.
By preference, the semiconductor device constitutes a vital part of a smart card, such as a cash card, a credit card or any other card. The expression xe2x80x9csmart cardxe2x80x9d is meant to describe in general any portable apparatus which comprises a semiconductor device that contains only data which cannot be changed by the apparatus alone. Although the smart card is a card-like apparatus, at present, the introduction of contactless smartcards makes it possible to incorporate the function of the smart card into other apparatus such as, for example, an electronic diary.
The second object of the invention, discussed above, may be achieved by providing a smart card comprising a semiconductor device in accordance with the teachings of the invention. Such a smart card will then be protected against visual inspection from an unwanted outside source. Preferably such a smart card comprises a semiconductor device that includes a light-sensitive element so as to protect the memory of the smart card against visual inspection after opening of the semiconductor device.